There are several DIY canoe dolly/trolley/cart plans via a Google search.
Are you sure you want to go this route though as there are many canoe carts for almost any price range. For example, we have a cart similar to
this one here that we picked up second hand for $40.00. I am sure if you looked on Ebay, there would probably be some that will fill your needs. Also you can search on the
Paddling.net classifieds section.